[Beowulf] water cooling

[Mark Hahn]

Hi all,
I'm involved in some planning that tries to evaluate large HPC datacenter
designs for a few years out.  One really fundamental issue that seems
unclear is whether direct water cooling will be fairly prevalent by then.

One train of thought is that power densities will increase to 30KW/rack or
so, necessitating water.  But will it be rack-back radiators (far less
efficient, but fairly routine today) or obtain much higher efficiency by
skipping the air-cooling step (like Aquasar, SuperMUC, K Machine, etc).

so, how commodity will direct water cooling be?
for extra points, what KW/rack density are you planning?

(by "commodity", I mean "available from vendors like HP/Dell/IBM as 
well as parts vendors like Supermicro.")

thanks, mark.